October 23, 2012

Black Hole Getting Ready For Some Cloud Lunch

Scientists have determined that the black hole in the middle of the Milky Way is getting ready to devour an approaching cloud of dust and gas.

The researchers used a supercomputer simulation to determine that the cloud, known as G2, could possibly survive the encounter with the black hole, but its surviving mass would be torn apart and shaped differently.

They came up with six simulations using Cosmos++ computer code, requiring more than 50,000 computing hours on 3,000 processors at Clemson University in Columbia, S.C.

Previous simulations of the event had been done in two-dimensions, but the new simulations included 3D capability, as well as a unique "moving mesh" enhancement. This enhancement allowed the simulations to more-efficiently follow the cloud's progression toward the black hole.

The composition of G2 is still a mystery. Astronomers originally found it in the region in 2002, but the first detailed determinations of its size and orbit came only this year.

The dust in the cloud has been measured at about 550 degrees Kelvin, which is about twice as hot as the surface temperature on Earth. The gas is about 10,000 degrees Kelvin, or twice as hot as the surface of the sun.

Astrophysicist Stephen Murray said that this black hole, Sgr A, is 3- to 4-million times as big as our sun, and has been relatively quiet recently, not feeding much.

As the cloud approaches Sgr A and begins to fall into it next September, G2 will begin to shred energy, causing it to heat to incredibly high temperatures, visible to radio and X-ray telescopes on Earth as well as orbiting satellites like NASA's Chandra X-ray observatory.

The point at which a stellar object cannot escape a black hole's grasp is known as the Schwarzschild radius, which is a quantity whose value depends on the black hole's mass, the speed of light and the gravitational constant.

The cloud will pass far enough away that it will escape the point of no return by about 2,200 Schwarzschild radii, or about 200 times as far as Earth is from the Sun.

Although it seems like far away, the supercomputer simulations show that the cloud will not survive the encounter.

"There's too much dynamical friction that it experiences through hydrodynamic instabilities and tidal stretching from the black hole. So a lot of its kinetic energy and angular momentum will be dissipated away and it will just sort of break up into some sort of incoherent structure," said computational physicist Peter Anninos.

He said much of the cloud will join the rest of the hot accretion disk around the black hole, or just fall and get captured by Sgr A.

"It will lose a lot of its energy but not all of it," Anninos added. "It will become so diffuse that it's unlikely that any remnant of the gas will continue on its orbital track."

The simulation shows clouds modeled as a simple gas sphere, near the point in its orbit where it was first discovered. As it approaches the black hole, it will be nearly five times longer than it is wide.

The cloud will experience resistance in the form of pressure as it tries to plow through the hot interstellar gas that already fills the space around Sgr A.